Amplifier



V. D. LANDON Aug. ll, 1936.

AMPLIFIER Filed May 17, 1923 INVENTOR Vernon D. Landon ATORNEY Patented Aug. ll, 1936 UNITED STATES AMPLIF IEE Vernon D. Landon, East Pittsburgh, Pa., assignor to Westinghouse Electric & Manufacturing Company, a corporation of Pennsylvania Application May 17, 1928, Serial No. 278,594

12 Claims.

My invention relates to amplifiers, and it has particular relation to amplifiers designed and intended to handle electric currents at radio-frequencies.

5 Substantially all amplifiers of the type to which my invention pertains embody the teachings of the Alexanderson Patent No. 1,173,079, in which is disclosed means for increasing the selectivity of a receiver by employing a plurality of therl mionic devices inter-connected in cascade through tuned transformers or tuned impedances. In order, however, to realize the full value of the amplification obtainable by tuned inter-tube couplng devices, it has previously 15 been found desirable and even necessary to pro vide means for preventing the amplifier from oscillatng and, consequently, interfering with the reception,of signals, when the coupling devices.

are tuned to approximate resonance with an in- 20 coming carrier wave.

Many arrangements have been proposed for preventing the spontaneous generation of oscillations in high-frequency amplifiers, the patent to Rice, No. 1,334,118 being followed, with some 25 variations, in the majority of such arrang'ements. Insofar as I am aware, however, no previously proposed anti-oscillation system has provided for eliminatng, at its source, the tendency toward regeneration, but al] of such systems have v9 included means, analogous to those disclosed by Rice, for neutralizing the said tendency after it has arisen.

In addition, the usual inter-tube tuned radiofrequency amplifier is quite critical, and great difiiculty is generally experienced in neutralizing it against oscillation. Such difiiculty, I have determined, results primarily from the highlyreactive character of the output circuits of the several thermionic tubes, and the consequent large feed-back occasioned thereby. The large feedback must be cancelled or neutralizedby correspondingly large anti-feed back devices which,

by reason of their size, cannot be easily adjusted to the critical values necessary.

Highly reactive plate circuits introduce yet another defect into the usual radio receiver which embodies tuned radiofrequency amplification, by reason of the fact that the reactance of the 50 plate-circuit inductors increases in proportion to the frequency being amplified. Even if such an amplifier is successfully neutralized against oscillation at the lower frequencies of the tuning range thereof, therefore, it will still have a pro- 55 nounced tendency, and, at times, an uncontrollable tendency, toward oscillation at the higher frequencies.

A further disadvantage of tuned radiofre quency amplifiers, constructed according to the teachings of the prior art, is their inability to discriminate equally well between two closely adjacent frequencies at opposite ends of the tuningrange covered. Since, with the present multiplicity of broadcasting stations, it is impossible to maintain even a ten kilocycle separation between 0 the carrier frequencies of al] stations within reach of a sensitive receiver, the lack of uniform selectivity of previous receivers is probably one of their most serious defects.

It is, accordingly, an object of my invention to provide a radio-frequency amplifier that shall have more nearly uniform selectivity over the broadcast-frequency range.

Another object of my invention is to provide a radio-frequency amplifier that shall be substantially incapable of oscillating over the entire tuning range thereof;

Another object of my invention is to provide a radio-frequency amplifier that shall exhibit unif0rm amplification characteristics over the tuning range thereof.

Another object of my invention is to provide in a radiofrequency amplifier of the aforementioned typ means for cntrolling the sensitivity thereof in order to compensate for manufacturing variations.

Still another and more specific object of my invention is to provide a multi-stage tuned radiofrequenoy amplifier wherein the Width of the resonance curve at high frequenciesis but little greater than the'width thereof at'low frequencies, and wherein the rotors of all of the tuning condensers may be maintained at ground-potential.

In applying my invention to a multi-tube 40 radio-frequency amplifier, I sochoose the constants of the inter-tube coupling transformers that, instead of presenting an inductive reactance to the preceding tube, each transf0rmer behaves, over the tuning range of the amplifier, as though it were substantially a pure resistance or a resistance in series,with a large condenser.

In a preferred embodiment of my invention, the inter-tube transformers are provided with dual-primary windings arranged in series-aiding relation to the secondary winding, insofar as radio frequencies within the range of the amplifier are concerned, and a condenser is connected in shunt to one of the said primary windings to ofler a decreasng reactance as the frequency being amplified increases. The inductive reaction of the unshunted primary winding increases with the increasing frequency, the increase being compensated in a manner that will hereinafter be specifically described.

In order that the amplifier shall not have a tendency toward oscillation, the two primary windings of the inter tube coupling transformers and the shunting condenser are so proportioned that the plate circuit of the tube with which any such transformer is associated has a capacity reactance over the entire tuning-range, when considered apart from the secondary winding of the transformer. When the said secondary winding is tuned, however, the capacitive reactance of the plate circuit may either be reduced, cancelled, or reversed in sign, as desired, depending on the amount of coupling between the primary and secondary windings.

My invention further consists in the proper proportioning of the constants of the amplifier stages preceding and following the stages in which are incorporated .my improved dual-winding transformers, and in the establishing of certain novel relationships among the several eircuits involved therein.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, asto its organization, its method of operation, and its underlying theory, will best be understood by reference to the following description of a specific embodiment, taken in connection with the accompanying drawing.

The single figure of the drawing is a schematic circuit diagram of a radio receiving system comprising a preferred embodiment of my invention.

The radio receiving system illustrated in the drawing comprising a plurality of thermionic devices l, 2 and 3 connected in cascade and arranged to amplify an incoming signal at radio-frequency, and a thermionic device 4 which functions as a detector.

The input circuit for the thermionic device l may comprise a variable resistor 5 which is included in series between an antenna 6 and a ground connection 1, or the said circuit may be arranged in any manner known to those skilled in the art since it constitutes no part of the present invention. If a resistor is utilized in the manner illustrated, the first thermionic device will be designated a blocking tube by those skilled in the art, and it will function primarily as a coupling means for transferring energy from the antenna to the radio receiver without, however, permitting the characteristics of the antenna-circuit from influencing the tuning of the said receiver.

The filaments of all the thermionic devices are preferably connected in parallel to a single poten tial source 8, and the platos of the said devices are supplied with operating potential from a high potential source IO which source may be a battery, as shown, or it may be any other source of unidirectional current. The grids of the first three thermionic devices are preferably biased to a negative, potential with respect to the filaments thereof, a, battery II being utilized for that purpose.

An inductor l2 is included in the output circuit of the blocking tube l, and is inductively coupled to an input inductor 13 connected between the. grid and filament of the first thermionic amplifier device 2. Atuning condenser M is connected in shunt to the input inductor l3 and serves to tune the input circuit.

A feed-back circuit, comprising an inductor 5 and a variable condenser [6, is provided for regeneratively transferring energy from' the output circuit of the thermionic amplifier device 2 to the input circuit thereof. The transfer of energy has an important result which will be referred to later.

An inductor l'l having a relatively small number of turns, and a second inductor l8, having a relatively large number of turns, are serially included in the output circuit of the second thermionic device 2, and both inductors are inductively coupled to a third inductor 20 which, in conjunction with a tuning condenser 2l connected in shunt thereto, comprises the input circuit of the third thermionic device 3. The second mentioned inductor 18 has a condenser 22 connected in shunt thereto, the condenser being of sufficient capacity to constitute, with the said inductor, a circuit tuned to a frequency below any frequency the amplifier is designed to handle.

For purposes of convenience, the coupling network, consisting of the three inductors, will be designated a dual-primary transformer.

The inductor l8 and the condenser 22 are, in a preferred embodiment of my invention, resonant to a frequency below 550 kilocycles and hence, for

an impressed voltage at a frequency within the present broadcast range; namely, 550 to 1500 kilocycles, the current in the condenser 22 will lead the voltage by while the current in the inductor l8 will lag 90 behind the voltage. current in the condenser will, consequently, be out of phase with the current in the inductor and will be the larger of the two currents. At the same time, the current in the small inductor IT will be the arithmetical difference between the currents in the inductor l8 and in the condenser 2 2 and it will be in phase with the current in the said condenser. Hence, if the inductors ll and l8 are wound in the same direction on a supporting form, as would naturally be the case in commercial apparatus, the connections to the large inductor must be arranged as illustrated in order that the effects from the two inductors I'l and l8 on the input inductor 20 shall be in phase.

The third thermionic device 3 and the fourth thermionic device 4 are coupled through a dualwindings in the output circuts of the thermionic devices 2 and 3, the said output circuits have, as a Whole, capacitive reactance to all frequencies higher than the frequency of the said tuned windings among which frequencies are included those lying in the present broadcast range. When, however, the secondary windings are tuned, it is found that the said capacitive reactance may be reduced, cancelled or reversed for a frequency closely adjacent to the frequency to which the secondaries are tuned, and that the plate cir The l 2,050,343: cuits may accordingly;be given the characteristics of a pure resistance,if desirable.

For all frequencies iu the tuning range, except the narrow frequency band being received, the impedance of the primary remains capacitive and, hence, damping is introduced into the preceding grid circuit. The frequency band for which the circuit is receptive lies slightly to one side of the frequency to which the secondary is tuned.

The efiect of this is to greatly augment the effective selectivity of the preceding grid circuit, since frequences lying outside of the desired band are damped.

It is a fact, well known to those skilled in the art to which my invention pertains, that a capacitive load in the plate circuit of a thermionic amplifier causes damping in the grid circuit of the device, while a resistive load has no effect thereon. This is perhaps the reason why, when my invention is applied to a radio-frequency amplifier, there is no tendency toward oscillation if only two tuned stages are employed. When three tuned stages are utilized, however, the reaction between the several stages becomes quite complicated, and it is diflicult to achieve the maximum predicted amplification without oscillation.

By reason of the reaction of the third tuned circuit on the second tuned circuit, the reaction Ofthe latter on the first tuned circuit is modified in some manner that I have not as yet positively determined. Irrespective of the underlying theory, however, it has been experimentally proved that, although it is comparatively easy to obtain an amplification of over the entire scale of frequencies if only two tuned stages are employed, an amplification of 400 with three such unmodified stages is substantially impossible to obtain without oscillation. r

In view of the aforementoned characteristics of an amplifying system in which are incorporated my improved dual-primary transformers, I have found it advantageous to so arrange the constants of the coupling network between the second and third tubes that the reactance of the plate circuit of the second tube is always capacitive, irrespective of the tuning of the input circuit of the third tube, and to provide the feedback condenser l6 and the feed-back inductor |5 for transferring sufiicient energy from the output circuit of the second tube to the input circuit thereof to compensate the damping introduced by the said capacitive reactance. The feed-back circuit has some tendency to over-cancel the damping introduced by the capacitive reactance at high frequencies, and to under-cancel it at low frequencies, causing the amplifyng system to have slightly better selectivity at the high frequency end of the tuning range than would be otherwise anticipated.

By reason of the necessary inclusion of an audiofrequency transformer primary winding 3| and a by'pass condenser 32 in the output circuit of the detector tube 4, it was found that the reactance of the said circuit was sufiiciently capacitive to introduce an undesirable amount of damping into the grid circuit thereof. An inductor 33 was accordingly included in the said output circuit, which inductor, together with the distributed capacity of the primary winding and the by-pass condenser 32, provides a circuit which is series-resonant to a frequency of approximately 1,200 kilocycles. The capacitive reactance of the output circuit of the tube ,4- is accordinglycancelled at the high-frequency end of the tuning range, with a corresponding cancellation of the damping of the grid circuit thereof.

Inasmuch as it is substantially impossible to commercially manufacture coils and condensers that are exact duplicates, the feed-back condens er Ii6 is ,preferablymade variable, thus permitting the sensitivity of the system to be adjusted to compensate for manufacturing variations before the apparatus is shipped from the factory.

An'amplifying system constructed according to.my invention has a peculiar characteristc that difierentiates it from other systems of the same general type with which I am familiar, namely, the tunable input circuits of the detector tube 4 and the high-frequency amplifier tube 3 immediately preceding. it must not be tuned exactly to the carrier-frequency of the desired signal for best results. Thev basis for this statement lies in the fact that the capacitive reactance of thedual-primaries is not cancelled for the frequency to which the secondaries are tuned, but for a frequency closely adjacent thereto. In order, therefore, that the full advantage of my improved system may be realized, it is preferable that the tuning condensers shall be so linked together for simultaneous actuation, as illustrated, that the input circuits l3-M and 2-2l shall always slightly differ in frequency from the circuit 2526. r

In the construction of a receiving set according to my inventon, it is highly important that the coupling of the small primary windings, as well as the coupling of the large primary windings, to the tuned secondaries of the dualprimarywinding transformers, shall be carefully adjusted. The coupling of the small primaries is varied to obtain the condition of a purely resistive plate circuit for high frequencies, while the coupling of the large primary windings to the secondaries is adjusted to control this condition at the lower frequencies of the tuning range. At frequencies lying toward the middle of the tuning range, the desired capacity cancellation results as a matter of course if the adjustments at each end of the tuning range are properly made. r

The theory underlying the action of my improved dual-windng transformer is extremely com;plicated and its mathematical treatment wouldadd but little to the present disclosure. It is considered advisable, however, to briefiy point out the manner in which my improved dualprirnarywinding transformer ders in function and operation from radio-frequency transformers of the usual type which comprise a primary winding having only a few turns in inductive relation to a tuned secondary.

In the usual radio-frequency transformer, at frequencies higher than that to which the secondary is resonant, a; capacitive reactance is introduced in series with the primary by the presence of the tuned secondary, while, at frequencies lower than the resonant frequency, an inductive;reactance is similarly introduced.

If another circuit, comprising an inductor shunted by a capacitor, is coupled to the tuned secondary, the frequency of maximum response of that secondary, to excitation from its primary, is shifted. If the added circuit is tuned to a lower frequency than the resonant frequency of the secondary, the point of maxmum response will be shifted to a frequency higher than that of resonance for the isolated secondary. The combination of the secondary with its coupled circuit now has an effect on the primary which is exactly equivalent to the effect of a solitary secondary tuned to the new frequency of maximum response. That is, at higher frequencies. a capacity is induced into the primary and at lower frequencies an inductance.

With the foregoing explanaticn in mind, the operation of my improved dual-primary-wnding transformer is quite clear. The resonant frequency of the secondary (isolated) is lower than the frequency of operation. Hence, at the frequency of operation, a capacity reactance is induced in series with the large primary, reducing this primarys reactance to a value nearly as low as the reactance of the shunt condenser.

However, the secondary, in combination with the coupled large primary circuit, constitutes a circuit tuned to a frequency considerably higher than the frequency of the secondary alone. The

frequency to which this combination tunes is also higher than the operating frequency. Hence, the effect of the combination on the small primary, at the operating frequency, is to induce an inductance in series therewith.

It, therefore, follows that a capacity is induced in series with the large primary at the operating frequency while an inductance is induced in the small primary. Both effects tend to lessen the capacitive nature of the output circuit of the preceding tube, considered as a whole.

Three frequencies have to be considered at each setting of the tuning condenser, namely:

1. The frequency of resonance of the isolated A secondary.

2. The frequency of maximum resonance of the secondary while coupled to the large primary with its shunt condenser.

3. The frequency of maximum tendency toward capacity cancellation in the primary circuit as a whole.

Since the frequency of maximum tendency toward capacity cancellation in one primary may diier from that of the other primary, the frequency of maximum combined effect is a compromise between the two effects.

Accordingly, although frequency "1 is always lowest of all, and frequency 3 is higher than frequency 2 at the high-frequency end of the scale, frequencies 2 and 3 may be about equal at the low-frequency end of the scale, where the effect of the small primary is very small.

In radio receiving systems utilizing transformers of the usual type, having a single primary winding inductively coupled to a single secondary winding, there is an inherent condition which is very undesirable, i. e., if suiicient coupling is employed to obtain the desired amplification at low frequencies, the coupling is so great at high frequencies as to very greatly detract from the selectivity at such frequencies. For this reason, it has been the custom to utilize less than the optimum value of coupling at low frequencies in order that the coupling at high frequencies shall not be too excessive. This results in a receiver which is too sharp at low frequencies and which has insuificient amplification at such frequencies. On the other hand, at high frequencies, the selectivity is poor, and the amplification, as well as the tendency toward oscillation, are excessive. By my invention, the coupling at high frequencies may be separately adjusted from that at low frequencies. In adjusting the couplings to obtain a purely resistive plate circuit, as hereinbefore described, the value of coupling obtained at high frequencies is less than that in the conventional radio-frequency transformer, and the coupling at low frequencies is greater. This gives rise to practically uniform amplification over the tuning range, and more nearly uniform selectivity. 5

In an actual receiver constructed according to my invention, the constante thereof are approximately as foll0ws: Resistor 5, 2,000 ohm; inductor l5, 30 microhenries; inductors |3, 280 microhenries; inductors 20 and 2B, 300 microhenries 10 each, inductors l8 and 24, 1,300 microhenries each; condensers 22 and 25, micro-microfarads each, inductor IZ, 30 microhenries; inductors l! and 23, 5 microhenries each; and condensers M, 2| and 21, a minimum capacity of 15 micro- 15 farads each, The small inductor 33 included in the output circuit of the detector tube has a value of 14 microhenries, and the by-pass condenser 32 is of the order of 1,200 micro-microfarads.

I appreciate, of course, that grounded rotors 20 per se are old and well known in the art, and I also am aware that grounded rotors have been utilized in neutralized receivers. It is my belief, however, that, by my invention, I have provided, for the first time, a radio receiver in which an 25 approach to uniform selectivity is obtained, and which is free from any substantial tendency toward oscillation, and, at the same time, in which all of the rotors of the tuning condensers are grounded. 30

Although I have chosen a single specific embodiment of my invention for purposes of illustration, it is not confined to receiving systems of the exact type shown, but is capable of broad application to receiving or amplifying systems of 35 any type wherein currents at high frequencies are handled if it is desirable to prevent the generation of oscillations therein. Many modifications of my invention will be apparent to those skilled in the art to which it pertains, and it is, accordingly, not to be limited except insofar as is necessitated by the prior art or by the spirit of the appended claims.

I claim as my invention:

1. In a radio-frequency amplifier, a thermionic device provided with an output circuit normally having capacitive reactance, and a thermionic device having a tunable input circuit so coupled to said output circuit that said output circuit assumes resistive characteristics at a frequency adjacent to any frequency to which said input circuit is tuned.

2. In a radio-frequency amplifier, a thermionic device provided with an output circuit having capacitive reactance over a specified range of frequencies, and a thermionic device having an input circuit tunable over said range, the said circuits being inductively coupled only, to permit energy transfer therebetween so that said output circuit displays resistive characteristics at a frequency adjacent to any frequency to which said input circuit is tuned.

3. In a radio-frequency amplifier, a thermionic device having an input circuit and provided with an output circuit having capacitive reactance over a specified range of frequencies, and a thermionie device having an input circuit comprising an induct0r and a variable capacitor for tuning it over the said range, the said circuits being inductively 70 coupled only, to permit energy transfer therebetween so that said output circuit acts as a resistance load at a frequency adjacent to any frequency to which said second mentioned input circuit is tuned and does not cause the feeding back 75 of arge anlunts of energy to said first-mentioned input circuit.

4. In a radio-frequency amplifier, a thermionic device having an input circuit and provided with an output circuit having capacitive reactance over a specified range of frequencies, a thermionic device having a tunable input circuit coupled to said output circuit, whereby said output circuit acts as a resistance load at a frequency adjacent to any frequency within said range to Which said second input circuit is tuned, and means for transferring energy at radio frequencies from the output circuit to the input circuit of said first-mentioned thermionic device in a sense to cause regeneration.

5. In a radio-frequency amplifier, a thermionic device having an output circuit comprising a pluraiity of inductors in series, a thermionic device having a tunable input circuit coup1ed to said inductors, and means in shunt to one of said firstmentioned inductors to constitute therewith a circuit tuned to a frequency lower than any frequency to which said input circuit is intended to be tunable.

6. In a radio-frequency amplifier, a thermionic device having an output circuit comprising a plurality of inductors in series, and means in shunt to one. of said inductors to constitute therewith a circuit tuned to a frequency lower than any frequency within the range of frequencies the ampiifier is designed to handle, the remaining constants of said output circuit being such that the output circuit has a capacitive reactance at any frequency in said range.

7. In a radio-frequency amplifier, a thermionic device having an output circuit comprising a plurality of inductors, means in shunt to one of said inductors to constitute therewith a tuned circuit, and a thermionic device having an input circuit tunable to higher frequencies than the frequency of said first tuned circuit, all of said inductors being inductively coupled together and said first mentioned inductors being connected in series-opposing relation for direct currents but in series-aiding relation for currents at frequencies higher than the frequency of said tuned circuit.

8. In a high-frequency amplifier, a thermionic device having a tunable input circuit and an output circuit comprising primary windings normally having capacitive reactance, means comprising a tunable secondary winding coup1ed to said primary windings to reduce the reactive impedance thereof at a frequency slightly different from the frequency to which said secondary winding may be momentarily tuned, and means for transferring energy from the output circuit of the ther, mionic device to the input circuit thereof out of phase with the energy transferred therebetween through the inter-electrode capacity of said device.

9. In a high-frequency amplifier, a thermionic device having a tunable input circuit, an output circuit therefor comprising primary windings normally having capacitive reactance, and means comprising a tunab1e secondary winding so coupled to said primary windings as to approximately cancel the reactive impedance there0f at a frequency near the frequency to which said secondary winding may be momentarily tuned.

10. A transformer having a plurality of primary windings, a condenser connected in shunt to one of said windings, a secondary winding, and a variable condenser connected in shunt to said secondary winding, the resonant frequency of said first condenser and its associated winding being lower than that of the frequencies to which the variable-condenser circuit is capable of being tuned.

11. A transformer having a plurality of primary windings, a condenser connected in shunt to one of said windings, a secondary winding, and a condenser connected in shunt to said secondary winding, the said primary windings being connected in series opposing relation, insofar as direct currents are concerned.

12. A transformer having primary windings adapted to be included in the output circuit of a thermionic device, a condenser connected in shunt to one of said primary windings, a secondary winding, a condenser connected in shunt to said secondary winding, the resonant frequency of said first condenser circuit being lower than that of said second condenser circuit whereby the impedance of a thermionic tube output circuit in Which said primary windings are included becomes resistive at a frequency approximating the frequency to which said secondary winding is tuned by the said shunting condenser.

VERNON D. LANDON. 

